Street games are predominantly physical games played in the streets, incorporating the built urban environment, spatial layout, social and political char- acteristics of urban sites into the gameplay. This paper outlines how rapid street game design and playing street games are means of knowledge generation for urban change. To develop the argument, it looks first at implicit aspects of design knowl- edge in an iterative design process. It then explores the role of explicit and implicit rules in game design as well as the concept of the magic circle that incorporates both the game design and the context of the actual urban site. Game design examples underpin the exploratory and prototyping aspects of street game design.